Constant beamwidth antenna

ABSTRACT

An antenna assembly for radio frequency energy is disclosed wherein individual antenna elements making up an array are fed in an improved manner so that the width of a directive beam formed by such elements is maintained substantially constant over a wide band of operating frequencies. The feed, here in a form equivalent to a horn, is made to have frequency dependent characteristics such that the amplitude taper across the aperture defined by the array is increased as operating frequency is increased.

United States Patent Shanafelt et al.

Archer et al 343/754 CONSTANT BEAMWIDTH ANTENNA 9 73 [75] Inventors:Robert E. Shanafelt; Richard F.

Hilton, both of Goleta; Donald H. Pr'mary Exammer l';h Llebefnan Archer,Santa Barbara, all of Calif. g g y Ageg, g l p J Joseph annone; 1c ar arans [73] Assignee: Raytheon Company, Lexington, y

Mass 57 ABSTRACT [22] Filed: 1974 An antenna assembly forradio frequencyenergy is [21] Appl. N0.: 442,703 disclosed wherein individual antennaelements making up an array are fed in an improved manner so that thewidth of a directive beam formed by such elements is 343/7154 g3 g i gzmaintained substantially constant over a wide band of [58] Fie'ld 54 909853 operating frequencies. The feed, here in a form equivalent to ahorn, is made to have frequency dependent [56] References Citedcharacteristics such that the amplitude taper across the aperturedefined by the array is increased as oper- UNITED STATES PATENTS atingfrequency is increased. 3,354,461 11/1967 Kelleher 343/854 3,755,8158/1973 Stangel et a1. .1 343/854 2 Clam, 2 Drawing Flgures US, PatentNov.18,1975 3,921,176

BEAM c BEAM B BEAM A AXIS 0F SYMMETRY CONSTANT BEAMWlDTl-I ANTENNABACKGROUND OF THE INVENTION This invention pertains generally todirective antennas for radio frequency energy and particularly towide-band directive antennas for radio frequency energy.

It is known in the art that an array of antenna elements may be fedthrough a parallel plate lens, i.e., a microwave lens, and a pluralityof transmission lines in such a manner that one, or more, beams of radiofrequency energy are formed. With proper design, such an assembly may beoperative over a wide band of frequencies, say an octave band. Becausethe principle of reciprocity applies, such an antenna assembly is alsoadapted to receive radio frequency energy Within the same frequency bandfrom one, or more, directions.

In one known antenna assembly of the type just mentioned, a designdefining a linear array of antenna elements, transmission lines,microwave lens and a plurality of feedports are formed on a commondielectric substrate using printed circuit techniques. After the soprinted dielectric substrate is assembled in operative relationship withone or two ground planes (depending upon whether a microstrip or astripline assembly is desired), constrained paths in the dielectricsubstrate are defined for radio frequency energy within a relativelywide frequency band. The dimensions of, and spacing between, the variousparts of the printed design determine the characteristics of thecompleted antenna assembly. In particular, with a plurality of feedportsalong a focal arc, the printed design is so arranged that the electricallengths of the paths between each feedport and the antenna elements aresystematically controlled. When all of the feedports are energized, thephase shifts experienced by radio frequency energy passing fromeachfeedport to the antenna elements are such that a plurality ofsimultaneously existing beams of radio frequency energy is formed, 'eachpointing in a different direction. The same antenna assembly may be'operated to form a single .one of the beams by simply energizing asingle one of the feedports. While such an antenna assembly is adaptedto operation over a wide band of frequencies, experience has proven thatthe beamwidth of its radiated beam, or beams, varies inversely withfrequency.

While a variation in beamwidth due to a change in operating frequencymay be tolerated in many applica- ,tions, cases exist where such avariation seriously affects proper performance. For example, if (whenthe antenna assembly is to produce a plurality of simultaneouslyexisting beams) it is desired to maintain the vpower level at thecrossover point between adjacent beams, any variation in beamwidth dueto a change in operating frequency obviously should be avoided.Similarly, if when the antenna assembly is to produce a single beam) itis desired to reduce clutter when a beam is pointed so as to graze anextended area, as the sea or a land mass, it is also obvious that anyvariation in beamwidth due to a change in operating frequency should beavoided.

SUMMARY OF THE INVENTION Therefore, it is a primary object of thisinvention to provide an improved antenna assembly adapted to produceone, or more, beams of electromagnetic energy, such beam, or beams,having a beamwidth which is sub- 92 stantially invariant over a wideband of operating frequencies.

This object, and other objects to be discerned, are

. achieved by providing, in a directional antenna of the type hereconsidered, a feedport, or a plurality of feedports, so dimensioned andpositioned that the amount of radio frequency energy passed toindividual antenna elements in a linear array is varied in a controlledmanner as operating frequency is changed. Such a controlled variation iseffective over a given band of operating frequencies to maintain theelectrical size (measured in wavelength) of the aperture defined by thelinear array of antenna elements at a substantially constant value.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding ofthis invention, reference is now made to the following description ofthe accompanying drawings wherein:

FIG. 1 is a diagram, greatly simplified, of an antenna assemblyaccording to this invention showing the manner in which such an assemblyis related to transmitters and receivers in a system, the illustratedantenna assembly being partially broken away to show details ofconstruction of such assembly; and

FIG. 2 is a plan view of the dielectric substrate of the antennaassembly of FIG. 1 showing the printed design of the various elements insuch antenna assembly and also illustrating how the amount of radiofrequency energy passing to each antenna element is controlled asoperating frequency is changed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before referring to thedrawings, it should be understood that the beam forming elements of ourpreferred antenna assembly are parts of a single stripline or microstripantenna assembly. That is, a printed circuit definingconstrained pathsfor electromagnetic energy is formed on one side of a dielectricsubstrate and a metallic ground is formed on the other side of suchsubstrate (if a microstrip antenna assembly is desired), or a dielectricslab is placed over the printed circuit with a second metallic groundplane covering the exposed side of the dielectric slab (if astriplineantenna assembly is desired). For convenience, then, thevarious parts of the printed circuit will be referred to as though theyare, in fact, complete elements. For example, the part of the printedcircuit in FIG. 2 defining a plan view of a feedport will be referred toas the feedport itself, it being understood that electromagnetic energyactually passes through the portion of the dielectric substrate (and thedielectric slab, if used) underlying the feedport.

templated antenna assembly 10 may be connected in a conventional mannerto a plurality (here three) of transmitters 14a, 14b, 14c and a likeplurality of receivers 16a, 16b,' through, respectively, transmit/-receive switches 18a, 18b, 180. The various transmitters and receiversare synchronized by a common system synchronizer 20 of conventionaldesign. Each one of the transmit/receive switches 18a, 18b, 18c isconnected to a different one of three feedports 22a, 22b, 220 throughlines (not numbered). The feedports 22a, 22b, 22c will be describedhereinafter; suffice it to say here that-each is designed to launchradio frequency energy through constrained paths in such a manner thatthe relative amount of such energy reaching each one of a plurality ofantenna elements 2411.....24/1 varies with operating frequency.

The feedports 22a, 22b, 220 are disposed along a focal arc andcontiguous with a microwave lens 26 which in turn is connected, throughmatching sections (not numbered) to a plurality (here eight) oftransmission lines 28a through 28/1 to define constrained paths forelectromagnetic energy to each one of the antenna elements2411.....24/1. For reasons discussed in detail in U.S. Pat. No.3,761,936 entitled Multi-Beam Array Antenna, issued Sept. 25, 1973,directive beams of electromagnetic energy then are formed, as shown,when all of the transmitters 14a, 14b, 140 are energized.

A detailed exposition of the reasons why the directive beams ofelectromagnetic energy from our antenna assembly remain substantiallyconstant in width as operating frequency is changed will now be made.For convenience, only the manner in which a broadside beam (Beam B inFIG. 1) is produced will be explained in detail, it being deemed obviousthat other beams are similarly produced. Referring now particularly toFIG. 2, it will be observed that the field pattern (sometimes referredto hereinafter as the primary illumination pattern) of electromagneticenergy from feedport 22bmay be caused to change as operating frequencyis changed. That is, the primary illumination pattern of theelectromagnetic energy from feedport 22b may be relatively broad (asindicated by the ine marked f,) at the lower end of an operatingfrequency band and relatively narrow (as indicated by the line marked atthe upper end of such band. It follows that, at the lower end of theoperating frequency band, the electromagnetic energy passing through themicrowave lens 26 from the feedport 22b to the various transmissionlines 28a.....28h may be directed so as to be almost equally betweensuch lines. This means that the amount of radio frequency energy passingfrom each of the antenna elements 24a.....24h is the same. To put itanother way, the amplitude taper across the aperture defined by antennaelements 24a.....24h at the lower end of the operating frequency band issuch that all such elements are substantially equally illuminated. Atthe upper end of the operating frequency band, the primary illuminationpattern, f,,, is such that a relatively greater amount of radiofrequency energy is passed to the centrally located ones of the antennaelements 24a.....24h than the end ones of such elements. It is apparentthen that the amplitude taper across the aperture defined by the antennaelements 24a.....24h is, in this case, such that the elements in thecenter of the array are illuminated and the elements at the edge of thearray are not illuminated. At any operating frequency intermediate f,and f,,, the primary illumination pattern produced by the feedport 22bis intermediate the illustrated patterns. That is, as the operatingfrequency is increased from f, toward f,,, the amount of amplitude taperacross the aperture defined by the antenna elements 24a.....24h iscorrespondingly increased. As a result, the width of beam B ismaintained substantially constant.

It has been found that the width of the feedport 22b, i.e., the distancebetween the points marked A and B in FIG. 2, is of primary importance ifthe primary illumination pattern is to be changed in a controlled mannerin accordance with a change in operating frequency. For example, in oneparticular design for an antenna to produce a nominal beamwidth of 20over an operating frequency band of 7 to 17 GHZ the optimum width of thefeedport 22b was found to be 1.2 inches. Expressed in wavelengths at theupper end of the operating frequency band (17 GHz), such width isapproximately equal to 1.73 wavelengths. At the lowr end of theoperating frequency band (7 6112), such width is approximately equal to0.75 wavelengths. At the midpoint of the operating frequency band (12GI-Iz) such width is approximately equal to 1.0 wavelengths. The ratioof the beamwidth of the resulting beam at 7 GHz to the beamwidth of theresulting beam at 17 GHz (with feedports 22a and 22c terminated inmatched loads) is 1.35 to 1. This contrasts with a beamwidth ratio of2.43 to l in a conventional design for a feedport having a width ofapproximately 0.35 inch (one-half wavelength at 17 GHz).

It will be noted here in passing that, if desired, the resulting beamfrom the antenna elements 22a.....22lz need not be broadside, but may beskewed at any desired angle within broad limits by incrementing, in aknown manner, the lengths of the transmission lines 28a.....28h andchanging the spacing between adjacent antenna elements to preventgrating lobes from being formed. Even though the resulting beam may beskewed, the primary illumination pattern from feedport 22b remainssymmetric about the axis of symmetry.

It will now be obvious that, if either feedports 22a, 220 are energized(along with feedport 22b to produce two or three directional beamssimultaneously), the primary illumination patterns from feedports 22a,220 will not be symmetric about the axis of symmetry. The asymmetricdisposition of such primary illumination patterns may permit somegreater change in beamwidth, as operating frequency is changed, than isexperienced in the case of a symmetric primary illumination pattern; anysuch greater change is, however, far less than would be experienced withfeedports having a width in the order of one-half wavelength. Theresult, then, is that the power level at the crossover points betweenadjacent beams remains far more nearly constant.

Having described a preferred embodiment of our invention, it will beapparent to one of skill in the art that our inventive concepts may beapplied to antenna assembies other than that illustrated. The idea thatan array of antenna elements may be fed in a manner such that theamplitude taper of electromagnetic energy may (to maintain beamwidth) bea function of operating frequency is obviously applicable to antennaassembies of the type illustrated regardless of particular operatingfrequency band or beamwidth required. Further, the same idea may beapplied to a space fed planar array antenna. It is felt, therefore, thatthis invention should not be restricted to its illustrated embodiment,but rather should be limited only by the spirit and scope of theappended claims.

What is claimed is:

1. In an antenna assembly for simultaneously providing a plurality ofoverlapping beams of electromagnetic energy, such beams being formed byinterference between electromagnetic energy passed through constrainedelectrical paths including printed circuit lens having an irregulargeometrical shape and radiated from an array of antenna elements, theelectromagnetic energy in each one of the plurality of overlapping beamshaving any frequency within a band of frequencies, the improvementcomprising:

a plurality, corresponding to the number of overlapping beams ofelectromagnetic energy to be radiated, of feedports for said lens, eachone of such feedports being responsive to electromagnetic energy havinga frequency within a desired band of frequencies, to form a likeplurality of beams of electromagnetic energy within said lens and eachone of such feedports having a width equal to the wavelength, at thecenter of the band of frequencies, of the electromagnetic energy. 2. Inan antenna assembly for providing a directive beam of electromagneticenergy, such beam being formed by interference between electromagneticensource.

1. In an antenna assembly for simultaneously providing a plurality of overlapping beams of electromagnetic energy, such beams being formed by interference between electromagnetic energy passed through constrained electrical paths including printed circuit lens having an irregular geometrical shape and radiated from an array of antenna elements, the electromagnetic energy in each one of the plurality of overlapping beams having any frequency within a band of frequencies, the improvement comprising: a plurality, corresponding to the number of overlapping beams of electromagnetic energy to be radiated, of feedports for said lens, each one of such feedports being responsive to electromagnetic energy having a frequency within a desired band of frequencies, to form a like plurality of beams of electromagnetic energy within said lens and each one of such feedports having a width equal to the wavelength, at the center of the band of frequencies, of the electromagnetic energy.
 2. In an antenna assembly for providing a directive beam of electromagnetic energy, such beam being formed by interference between electromAgnetic energy radiated from an array of antenna elements energized, through constrained electrical paths including a printed circuit lens having an irregular geometrical shape, by electromagnetic energy from a source thereof, such source being adapted to produce electromagnetic energy at any frequency within a band of frequencies, the improvement comprising: Means coupling a source of electromagnetic energy to said lens, such means including a feedport having a width equal to the wavelength, at the center of a band of operating frequencies, of the electromagnetic energy out of such source. 